1. Field of the Invention
The field of the invention is related to gyroscopes of the electrostatic pickoff type and, more particularly, in the provision of an all-attitude (wide angle) pickoff for determining the attitude of the ball spin axis relative to the gyroscope case.
2. Description of the Prior Art
Gyroscopes exist in the prior art in which a spinning ball is supported or levitated in a high vacuum by electrostatic forces. For this purpose, typically three or more opposed pairs of spherical-surfaced electrodes are provided in a closely fitted cavity and circuit means are provided such that, if the ball becomes uncentered, i.e., recedes from one electrode of a pair and advances towards the other, the electrostatic force is increased and decreased, respectively, at the two electrodes, urging the ball towards center. For an example, see U.S. Pat. No. 3,334,949, issued Aug. 8, 1967, entitled "Electrostatic Bearing" by James L. Atkinson, one of the inventors of the present invention. In some cases, the same electrodes perform sensing and forcing functions; in others, for example, U.S. Pat. No. 3,482,455, issued Dec. 9, 1969, entitled "Electrostatic Levitation Control System" by J. C. Boltinghouse et al, different electrodes are used for sensing and forcing.
In using an electrostatic ball gyroscope for many purposes in flight control and in inertial navigation, it is desirable that the case be fixed to the frame of the carrying craft (ship, aircraft, missile, etc.) as distinguished from being mounted on a three-axis stabilized platform. Such manner of use does away with the complex mechanical elements and gimbal servo systems of a stabilized platform. In frame-fixed, so-called "strapdown" systems, the ball spin axis remains angularly fixed in inertial space while the craft may assume any arbitrary attitude (angular orientation).
In such strapdown systems, the problem is presented of accurately measuring the attitude of the gyro case, which is fixed to the craft frame relative to the ball spin axis. For practical purposes, the accuracy of sensing of the spin axis must be of the order of one milliradian (3.4 minutes of arc) or better. One class of methods and apparatus which has been proposed for spin axis sensing is the optical class. Some sort of optical pattern is put on the ball and a plurality of optical pickoffs are provided, sensing the changing orientation of the pattern. A typical system of this sort is U.S. Pat. No. 3,154,953 issued Nov. 3, 1964, entitled "Gyroscopic Control Apparatus," by R. D. Ormsby. Such systems suffer certain disadvantages. First, the optical system is cumbersome and bulky. Second, it is exceedingly difficult to provide a pattern on a sphere, which may be only a centimeter or so in diameter, which yields sufficient angular resolution. Third, even given a fine enough pattern, it is almost impossible to locate it accurately enough relative to the actual spin axis of the ball. Until the present invention, no really practical all-attitude pickoff for electrostatic ball gyros has been devised.